A number of modern semiconductor wafer fabrication processes require a process gas to be supplied in a carefully controlled manner to a reaction chamber, wherein the process gas is used to support or effect processing of the semiconductor wafer. For example, in a plasma etch process, a process gas is supplied to an etching chamber, wherein the process gas is converted to a plasma for etching materials present on the surface of the wafer. In most cases, the semiconductor wafer fabrication processes require the process gas supply to be carefully controlled. More specifically, a flow rate of the process gas to the reaction chamber needs to be maintained within a range defined by a recipe of the fabrication process. The process gas flow rate is commonly controlled by a mass flow controller (MFC) upstream from the reaction chamber. Thus, the accuracy at which the process gas flow rate can be controlled is generally dictated by the accuracy of the MFC through which the process gas is required to pass.
It should be appreciated that the MFC device is a complex and sensitive instrument having a real-world gas flow rate control accuracy that is dependent upon many factors. During manufacture of the MFC device, the gas flow rate control provided by the MFC is verified to be within established MFC design specification tolerances. The MFC verification during manufacture is typically performed in a controlled laboratory environment using N2 gas. Thus, the MFC verification during manufacture may not bound the environmental conditions to which the MFC will be exposed during a real-world implementation. Additionally, conversion factors are used to translate the MFC verification results using N2 into corresponding verification results representing a real gas. It should be appreciated that these conversion factors have an inherent level of uncertainty. Furthermore, after the MFC device is shipped to the end-user and installed in the end-user's system, a potential exists for the MFC device to be out of tolerance with respect to its design specification. Also, the gas flow rate control capability of the MFC device needs to be periodically verified to ensure that an out of tolerance condition has not be introduced in the form of calibration drift, zero drift, or gas-calibration error that may occur during startup or service of the MFC device.
In view of the foregoing, it is desirable to verify the gas flow rate control capability of the MFC device in the real-world implementation using real gases. However, the end-user's equipment for verifying the gas flow rate control accuracy of the MFC device is typically not capable of matching the tight tolerance levels of the MFC design specification. Therefore, a need exists for improvements in technology related to accurate verification of the gas flow rate control capability of the MFC device under anticipated operating conditions.